Conventional disk drive assemblies typically include one or more disks, which include a plurality of concentric tracks that are radially displaced from each other on the surface of the disk for storing data. During disk fabrication, servo data is written on the disk by a servowriting process to delineate the centerlines of the tracks. During subsequent disk operations, the servo data is also read by a read/write head to provide information regarding the position of the head with respect to the track. The head position information enables a servo controller to re-align the head over a track when position errors are detected.
During the servowriting process, the servo data is written on the disk as a plurality of servo samples that are radially displaced from one another about the disk. Each sample comprises at least two servo bursts of constant amplitude and frequency (magnetic, reflective, or otherwise) that are recorded on the disk as sequential fields. Typically, the servo bursts are offset from the centerline of track in some particular pattern. One conventional burst pattern is a two-burst pattern, where "A" and "B" bursts are written symmetrically offset from and on respective sides of a track centerline. Another conventional burst pattern is a quad-burst pattern that includes a quad of servo bursts in each servo sample, typically designated as "A", "B", "C", and "D" bursts.
Ideally, the servowriting process would produce tracks that form perfectly concentric annuli about the center of rotation of the disk spindle, and that are spaced at desired track pitch across the disk. Unfortunately, factors such as mechanical vibrations that are asynchronous to disk rotation during the servowriting process, disk defects, and edge/transition noise cause the tracks to form irregular concentric paths and to deviate in track pitch. The track errors produced on the disk are measured as Repeatable Runout (RRO).
The asynchronous component of RRO caused by radial vibrations and motions that occur between the head and disk are asynchronous with disk rotation and asynchronous between adjacent tracks. This results in the track pitch variation which directly contributes to soft and hard errors. Additionally, a noise component of the RRO hinders the detection of events that should inhibit the writing of data onto the disk to prevent further errors.
The RRO is manifested as position error signals (PES) during subsequent disk operations. That is, as the disk rotates and the head samples the servo data, the position of the head relative to the track is measured as position error. The position error signals are input to a servo controller which then uses the signals to generate compensation signals to re-align the head over the centerline of the track. Because the calculation of a position error signal during head tracking uses the boundaries between the servo bursts to define a measured position, and the servo bursts are written with inherent distortion, there is a one-to-one relationship between the "error" in the head-to-disk relative position when a servo burst was written and the "error" of the detected head position during tracking. Due to the presence of RRO errors on the disk, the servo controller may be prevented from correctly maintaining the position of the head over the desired centerline of the track. Accordingly, what is needed is a system and method for minimizing position errors on servo disk drives. The present invention addresses such a need.